docs: def buildprior(self, prior, mopt=None):
          """ Create prior to fit models in list ``models``. """
          nprior = gvar.BufferDict()
          for m in self.flatmodels:
              nprior.update(m.buildprior(
                  prior, mopt=mopt,
                  ))
          if not self.fast:
              for k in prior:
                  if k not in nprior:
                      nprior[k] = prior[k]
          return nprior